This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-296823, filed Sep. 28, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor light-emitting element and a method of manufacturing the same.
In recent years, proposed are various semiconductor light-emitting elements using an InGaAlP series material having light-emitting characteristics in the visible wavelength region. FIG. 7 is a cross sectional view showing as an example the construction of an LED (light emitting diode) using a conventional InGaAlP series material and producing a visible light output. As shown in the drawing, a double hetero structure including an active layer 53 and p-type and n-type cladding layers 54, 52 having the active layer 53 sandwiched therebetween is formed on an n-type GaAs substrate 50. Also, an electrode 57 is formed on the p-type cladding layer 54 and an electrode 58 is formed on the lower surface of the n-type GaAs substrate 50.
It is necessary to select most suitably the band gap of each of the layers 52 to 54 forming the double hetero structure in accordance with the design in order to obtain a desired wavelength of the emitted light and to confine carriers. Also, it is desirable for the lattice constant of each of the layers 52 to 54 to conform with the lattice constant of the substrate 50 in order to achieve a satisfactory epitaxial growth of the layers 52 to 54. It should be noted that InGaAlP, which is a III-V compound, contains III group elements of In, Ga and Al. Therefore, it is possible to design independently the band gap and the lattice constant of InGaAlP by suitably selecting the composition ratio of these III group elements of In, Ga and Al.
It is possible to select the wavelength of the emitted light within the visible light region when the InGaAlP series double hetero structure is formed by the layers 52 to 54. Also, it is possible to form the InGaAlP series double hetero structure by the epitaxial growth having a lattice alignment with the GaAs substrate that is the most general compound semiconductor substrate. It is possible to obtain easily the GaAs substrate 50, and the epitaxial growth layers 52 to 54 can also be formed relatively easily. However, the GaAs substrate 50 is defective in that the transmittance of the light of the visible region is low. As a result, the light emitted from the InGaAlP series double hetero structure 53 to 54 is partly absorbed by the GaAs substrate 50, with the result that it is unavoidable for the brightness of the LED to be lowered.
In order to avoid the decrease in the brightness, it is necessary to use a material transparent to the light of the visible region for forming the substrate. In general, GaP is known as a transparent material. However, a lattice alignment cannot be achieved between the GaP substrate and the InGaAlP series material, resulting in failure to achieve a satisfactory epitaxial growth.
In order to avoid the difficulty noted above, proposed is a wafer bonding method in which the InGaAlP epitaxial growth layer and the GaP substrate are subjected to the wafer bonding. In this method, an epitaxial layer is grown first on the GaAs substrate, followed by removing the GaAs substrate from the grown epitaxial layer and subsequently attaching a GaP substrate prepared in advance to the epitaxial growth layer. In this case, the resultant structure consisting of the epitaxial growth layer and the GaP substrate is subjected to a heat treatment while applying pressure to both the epitaxial growth layer and the GaP substrate so as to make the structure integral.
It is certainly possible for the wafer bonding method described above to improve the brightness of the LED. However, it is difficult to handle the epitaxial growth layer after removal of the GaAs substrate because the epitaxial layer is thin. Also, a special apparatus is required because a heat treatment is applied while applying pressure to the structure consisting of the GaP substrate and the epitaxial growth layer. Such being the situation, a problem remains unsolved in terms of the stability and the productivity of the wafer bonding process.
On the other hand, concerning the wafer bonding method, developed is a technology called a direction adhesion or a direct bonding in which wafers each having a clean surface are bonded to each other. For example, a direct bonding of silicon wafers is disclosed in Japanese Patent No. 1420109, filed in 1983. Also, a direct bonding of compound semiconductor wafers is disclosed in Japanese Patent No. 204637, filed in 1985.
The light emitting efficiency of the LED manufactured by applying the bonding technology noted above is about twice as high as that of the conventional LED that does not employ the bonding technology in the manufacturing process and, thus, the LED manufactured by applying the bonding technology is called a high brightness LED.
On the other hand, it has been clarified that the brightness of a high brightness LED is rendered markedly nonuniform depending on the product in the high brightness LED of the bonding type, compared with the conventional LED. The reason for this problem is as follows.
FIG. 8 is a cross sectional view showing the construction of an LED prepared by the direct bonding method described above. As shown in the figure, a so-called n-up structure consisting of a p-type cladding layer 54, an active layer 53, an n-type cladding layer 52 and an n-type current diffusion layer 51, which are laminated in the order mentioned as viewed from below in the drawing, is mounted to a p-type substrate 60 with adhesive layers 61 and 55 interposed therebetween. Where Zn is used as a p-type dopant, Zn is contained in each of all the layers ranging between the active layer 53 and the substrate 60. Particularly, in order to decrease the series resistance of the LED, it is necessary for a substrate having a high impurity concentration, e.g., an impurity concentration not lower than 1xc3x971018 cmxe2x88x923, to be used as the p-type GaP substrate 60. It should be noted that Zn contained in the p-type GaP substrate 60 and the InGaAlP epitaxial growth layers 54, 55, and 61 is diffused into the active layer 53 in the heat treating step after the direct bonding step. Zn diffused in the active layer 53 forms an impurity level in the active layer 53. Since the impurity level acts as a non-light-emitting center relative to the current injected carriers, the density of the non-light-emitting centers is increased with increase in the amount of Zn diffused into the active layer 53. It follows that the injected carriers are caused to disappear without passing through the route of light emission/recombination. As a result, the brightness of the LED chip is markedly lowered.
The situation pointed out above is described in detail in, for example, (Jpn. J. Appl. Phys Vol. 33 (1994) pp. L857 to L.859 xe2x80x9cEffect of Substrate Micro-orientation and Zn Doping Characteristics on Performance of AlGaInP Visible Light Emitting Diodexe2x80x9d and Solid State Electron Vol. 38, No. 2, PP. 305 to PP. 308, 1995 xe2x80x9cAlGaInP ORANGE LIGHT EMITTING DIODES GROWN ON MISORIENTED p-GaAs SUBSTRATESxe2x80x9d).
The amount of Zn diffused into the active layer 53 is determined by the temperature and time of the heat treatment and by the amount of Zn held in the p-type layers acting as a diffusion source. Among these factors, it is possible to control the temperature and time of the heat treatment. The holding temperature of the actual heat treatment falls within a range of between 700xc2x0 C. and 770xc2x0 C., and the holding time is 1 hour.
It follows that, in order to control the amount of Zn diffused into the active layer 53, it is necessary to set constant the Zn concentration in each of the p-type GaP substrate 60, and the epitaxial layers 54, 55 and 61. Particularly, the GaP substrate 60 is thick and has a high carrier concentration, compared with the p-type epitaxial layers such as the cladding layer 54 and the adhesive layer 55. It follows that, in order to suppress the decrease and fluctuation in the brightness of the LED chip by suppressing Zn diffused into the active layer 53, it is considered highly effective to decrease the carrier concentration and to increase the stability of the Zn concentration in the GaP substrate 60. However, the use of a p-type GaP substrate of low and stable in impurity concentration leads to an increase in the series resistance of the LED chip. In addition, the cost of such a substrate of particular specification is high.
As described above, it is possible to increase the brightness of the conventional LED by directly bonding a GaP substrate to the InGaAlP series double hetero structure. In this structure, however, Zn used as a p-type dopant of the GaP substrate is diffused into the active layer so as to form a non-light-emitting center, with the result that the brightness of the LED is lowered.
The problem described above is inherent in not only the LED but also in a semiconductor laser constructed as shown in FIG. 9. In the semiconductor laser shown in FIG. 9, an n-type cladding layer 71, an active layer 72 and a p-type cladding layer 73 are formed in the order mentioned on a GaAs substrate 70. Further, an upper p-type cladding layer 75, a current facilitating layer 77 and a p-type GaAs layer 78 are laminated in the order mentioned on the p-type cladding layer 73 with an etching stopper layer 74 interposed between the p-type cladding layer 73 and the p-type cladding layer 75. Further, an n-type GaAs layer 76 is formed to surround the upper p-type cladding layer 75 and the current facilitating layer 77.
In the semiconductor laser of the described construction, the p-type GaAs layer 78 is formed by an epitaxial growth in direct contact with the n-type GaAs layer 76. In order to decrease the series resistance of the semiconductor laser, the p-type GaAs layer 78 is doped with Zn in a carrier concentration of about 2xc3x971018 cmxe2x88x923. However, the carrier concentration is actually fluctuated such that the carrier concentration is rendered higher or lower. Where the carrier concentration is fluctuated so as to be increased to exceed 2xc3x971018 cmxe2x88x923, the problem has been clarified that a large amount of Zn diffused from the p-type GaAs layer 78 into the active layer 72 through the current facilitating layer 77 during the power supply to the chip forms a non-light-emitting center so as to increase the value of the threshold current of the laser oscillation. Presently, the sole measure against the Zn diffusion is to control precisely the doping, with the result that it is unavoidable to manufacture an epitaxial wafer by a process of a very small margin. It follows that it is difficult to manufacture a semiconductor laser satisfactory in the light emitting efficiency with a high yield.
According to an aspect of the present invention, there is provided a semiconductor light-emitting element, comprising a double hetero structure formed of III-V group compound semiconductor layers including an active layer acting as a light emitting layer and an n-type cladding layer and a p-type cladding layer having the active layer sandwiched therebetween; a p-type layer laminated on the double hetero structure and containing Zn as a dopant; and a Zn diffusion preventing layer interposed between the active layer of the double hetero structure and the p-type layer.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor light emitting element by applying a wafer direct bonding technology, comprising forming a double hetero structure formed of III-V group semiconductor materials and including an n-type cladding layer, an active layer acting as a light emitting layer and a p-type cladding layer, which are laminated in the order mentioned, on a first substrate; forming a Zn diffusion preventing layer on the p-type cladding layer included in the double hetero structure; directly bonding a second substrate transparent to a light emitted from the active layer to the Zn diffusion preventing layer; and removing the first substrate.